Why is it in the News?

Tamil Nadu has battled heavy rainfall throughout December. At the beginning of the month, parts of Chennai and its neighbourhood experienced massive flooding because of Cyclone Michaung.

News Summary:

• Tamil Nadu has experienced significant rainfall, recording 450mm since October 1.
• Among the 38 districts, only 14 have reported deficient rainfall until December 20.

Seasonal Rainfall Norms:

• December rainfall is typical for Tamil Nadu, with the northeast monsoon being crucial for the region.
• Nearly 48% of the annual rainfall (443.3mm) in Tamil Nadu occurs from October to December, impacting rabi cultivation.

Recent Rainfall Patterns in Southern Tamil Nadu:

• Three districts in southern Tamil Nadu witnessed 'exceptionally' heavy rainfall from December 17 to 19.
• The India Meteorological Department (IMD) reported a 100% or more surplus during this period compared to the previous week (December 6-13, 2023).

Factors Contributing to Record Rainfall:

• Vigorous northeast monsoon over Tamil Nadu and Kerala, particularly in the southern regions.
• Development of a cyclonic circulation in the southwest Bay of Bengal on December 16, enhancing northeast monsoon winds.
• The persistence of this system over southern Tamil Nadu on December 18 and 19, led to heavy cloud convection and hefty rainfall.

IMD's Forecast:

• The cyclonic circulation has moved away from the Indian landmass, currently positioned over the southeast Arabian Sea.
• No significant rainfall is expected in Tamil Nadu, but the IMD predicts light to moderate intensity rainfall (up to 64mm in 24 hours) in certain areas of southern Tamil Nadu.

What is Northeast Monsoon?

• The northeast monsoon stands as a significant and permanent element within the Indian subcontinent's climate system.
• Its nomenclature originates from the prevailing direction of the monsoon winds—blowing from the northeast to the southwest.
• In their trajectory, these winds accumulate moisture from the Bay of Bengal, subsequently depositing it over southern states such as Tamil Nadu, Andhra Pradesh, Kerala, south Telangana, and Karnataka.
• This monsoon is alternatively known as the winter monsoon, retreating monsoon, or reverse monsoon.
• Period: The northeast monsoon maintains activity throughout the three-month span from October to December.
• Causes and Influences: One of the primary instigators of the northeast monsoon is the southward shift of the Inter Tropical Convergence Zone (ITCZ).
• The ITCZ, a dynamic belt near the Equator, marks the convergence of trade winds from the northern and southern hemispheres.
• The southward displacement of the ITCZ, coupled with the warming of the Indian Ocean, induces a reversal in the lower-atmosphere moisture-laden winds' direction—from southwest to northeast—thereby triggering the Northeast Monsoon (NEM).